1. Field of the Invention
The present invention relates to a document scanning device having a pair of light guides that extend in a main scanning direction of a document and illuminate the document, and a reference plate that is positioned closer to the light guides than the document.
2. Description of Related Art
In a document scanning device for scanning an image on a document, a lighting device illuminates the document face, and a reading sensor receives light reflected by the document face and outputs an image signal. Conventionally, such a lighting device generally uses a fluorescent tube as its light source. In recent years, however, LEDs have become common, in view of energy conservation, etc.
A number of LEDs do not necessarily have to be arranged over a reading area of the document. Just one LED can be used with a columnar light guide that extends over the entire reading area and guides light emitted by the LED to the document surface. Such a light guide has prisms to reflect the light emitted by the LED (Related Art 1).
A light amount of the light source fluctuates due to starting characteristics or a change in luminous efficiency caused by ambient temperature. Accordingly, when the light source is turned on or scanning of a document is started, a correction of the output image signal is performed on a basis of the light amount of the light source. In order to determine the light amount, a reference plate (usually a white sheet) is provided outside the document reading area to reflect light emitted by the LED toward the reading sensor (Related Art 2).    Related Art 1: Japanese Patent Application Publication No. 2001-61040    Related Art 2: Japanese Patent Application Publication No. 2006-211526
As shown in FIG. 7A-1, a pair of light guides 71 are used to illuminate a document A, and each light guide 71 has narrow prisms to avoid an unnecessary light illumination outside an area of a document reading width along a sub-scanning direction.
Meanwhile, as shown in FIG. 7A-2, a reference plate 73 is provided inside a document glass 72 to avoid interference with the document A and an adhesion of dust. Therefore, the reference plate 73 is positioned closer to the light guides 71 than the document A.
Accordingly, when the prisms of the light guides 71 are just wide enough to illuminate the document over the reading width, the center of the reference plate 73 is not illuminated at all. Thus, a sufficient amount of light is not reflected on the reference plate 73, and the light amount of (i.e. emitted by) the light source is not accurately determined.
On the other hand, as shown in FIGS. 7B-1 and 7B-2, when fluorescent tubes 74 are used instead of the LEDs and the light guides 71, the document A is illuminated over a wide area along the sub-scanning direction, and the whole reference plate 73 is illuminated.